Gaskets are often used as a seal between mating mechanical components. One common application involves gasket placement between the engine block and cylinder head of an internal combustion engine. The engine block and cylinder head are bolted together and the gasket relies on the force of the bolted connection to seal the various openings between the two mating components. In particular, cylinder head gaskets typically extend around cylinder bores to provide a combustion seal, maintaining the high temperature gases of combustion within the cylinder bores. Simultaneously, the gaskets also seal fluid flow openings such as coolant and oil openings to prevent undesirable mixing.
It is typical for a cylinder head gasket to include a main gasket body with a cylinder bore opening, the periphery of which is surrounded by a metallic generally U-shaped flange. The flange engages both the upper face and the lower face of the main gasket body. The flange provides improved protection to the gasket body from the high temperature gases of combustion and serves to dissipate the heat of combustion into the gasket body and away from the cylinder bore opening.
In some applications the main gasket body is metallic. However, a metallic main gasket body has a limited thermal conductivity. Thus, it is known to use composite materials with improved thermal conductivity. However, many of these composite materials have a reduced ability to withstand the stresses and strains induced by the bolted connection of the mating components. Yet, the flange relies on the main gasket body for support. As a result, the flange is subjected to greater dynamic stresses induced through the use of a main gasket body formed from composite materials and may fail over time in a form of combustion seal fatigue failure commonly known as "flange cracking". A cracked flange, taken in isolation, does not necessarily result in a failed combustion seal. However, if the crack becomes large enough to allow the composite material of the main gasket body to extrude out, the likelihood for the loss of sealing stress and a blown cylinder head gasket greatly increases.
The phenomenon of flange cracking has been of particular concern with graphite based composite main gasket bodies. Graphite has been the optimum material for use in a variety of composite head gasket designs. It includes improved conformability, heat resistance and relaxation properties when compared to other asbestos replacement materials. Yet, it has a very low shear strength which allows for the thermally induced lateral motion of the cylinder head and engine block to drag the flange laterally back and forth. Further, graphite also includes a relatively low spring rate which can allow for greater dynamic lift-off deflections at the cylinder bore opening. Unfortunately, the low spring rate remains almost constant even after prolonged exposure to heat. Therefore, the dynamic stresses are unable to decrease over time.
Prior art methods for combating flange cracking have typically focused on the base material of the metallic flange. In particular, it is known that flange fatigue strength can be increased by changing the base material from a low carbon steel to a stainless steel. Despite the cost penalty associated with using a stainless steel flange, experience has demonstrated that flange cracking is not fully eliminated.
Low friction coatings, typically molybdenum and Teflon.RTM. based, applied to the flange surface have also been sporadically attempted in the past with limited success. The belief was that such coatings would help reduce shearing stresses. However, known coatings are applied to the base material of the flange before the flange is formed into its final shape. As a result, the coatings must be soft or formable enough to withstand the flange formation process. The use of formable coatings reduce long term fretting resistance and the coatings eventually break down. Thus, flange cracking is postponed, but not eliminated.